Conventionally, carbon monoxide is obtained during the steam reforming or partial oxidation of hydrocarbons. It is possible to produce highly pure carbon monoxide with such units by using conventional purification techniques. These include cryogenic processes such as washing with liquid methane or partial condensation. Washing with methane makes it possible to produce highly pure carbon monoxide with, as by-product, impure hydrogen containing typically 1 to 2% methane.
This separation method makes it possible to obtain hydrogen under pressure and a very good carbon monoxide yield (up to 99%).
Partial condensation also makes it possible to produce highly pure carbon monoxide, as described in EP-A-0677483.
However, the carbon monoxide yield is generally not greater than 80% because of the losses with the hydrogen produced.
In order to increase the carbon monoxide yield, it is known in the art to lower the temperature of the condensation pot by reducing the pressure of the hydrogen flow escaping from the pot head. Another solution consists in recycling the impure hydrogen produced as a feed gas for the partial condensation unit.
These techniques are, however, expensive both in terms of the energy needed and investment.
In order to produce hydrogen with a purity suitable for the production of an ammonia synthesis mixture, cryogenic purification by washing with liquid nitrogen is conventionally used, as described in EP-A-0307983 and DE-A-3814293.
With this technique, pure hydrogen can be obtained from the synthesis gas output by a process of reforming or partially oxidizing hydrocarbons or coal.
Such a process is illustrated in FIG. 1 and will be described in brief below:
After removal of the soot at 1, and removal of the hydrogen sulphide at 2, from the mixture output by the step of partially oxidizing hydrocarbons or coal at 3, step 4 of converting the CO changes the majority of the CO present, under the action of steam, into carbon dioxide (CO.sub.2) while producing hydrogen. After removal of CO.sub.2 by absorption at 5, the gas mixture is subjected to a step 6 of washing with nitrogen.
The washing with nitrogen provides hydrogen under pressure at the column head, which has a high purity suitable for its use in the production of a synthesis mixture (N.sub.2 +3H.sub.2), as well as a residual gas containing essentially carbon monoxide, hydrogen, methane and nitrogen.
FIG. 1 also represents the other operations of the ammonia synthesis; air distillation at 7, providing the oxygen needed for the partial oxidation 3 and the nitrogen needed for the washing 6; NH.sub.3 synthesis proper at 8, from the synthesis mixture output by step 6, compressed at 9; steam expansion in a turbine 10 to drive the compressor 9; treatment of the hydrogen sulphide output by step 2, at 11, by a Claus process; and various heat exchanges 12 to 14.
In order to produce carbon monoxide in the particular case above, it is known in the prior art to treat the residue output by the nitrogen-washing step 6.
The residual gas is then used as the feed mixture for the carbon monoxide production process.
Such processes are described, in particular, in documents EP-A-00092770 and EP-A-0676373.
For the production of carbon monoxide by utilizing the by-product hydrogen, that is to say producing in parallel hydrogen under pressure with a purity sufficient for the preparation of a synthesis mixture, it has been proposed in the prior art to combine, with the step of washing with liquid methane for obtaining highly pure carbon monoxide, a subsequent step of washing with liquid nitrogen for purifying the hydrogen intended for the production of the synthesis mixture (N.sub.2 +3H.sub.2).